Firetube heat exchanger

ABSTRACT

A firetube having an elongated cylindrical shell is characterized by a plurality of circular rows of elongated U-shaped fins, each having a bottom surface, preferably curved along a radius or flat, secured to the inner surface of the cylindrical shell and two flat, planar, preferably parallel sides extending upwardly from the bottom surface, with the fins in each row aligned substantially parallel along the axis of the cylindrical shell.

BACKGROUND OF THE INVENTION

Firetube heat exchangers are well known for converting heat from hot gases of combustion to a material, typically a liquid, exposed to the outside surface of the firetube. Such heat exchangers are described in U.S. Pat. Nos. 5,913,289 and 6,675,746. These as well as other previously described firetube heat exchangers have been relatively expensive or difficult to manufacture. In addition, some firetube heat exchangers have been less effective in transferring heat from the hot gases of combustion passing through the interior of the firetube to the outside surface for heating the liquid. It is to an improved, highly efficient, and relatively economically manufactured firetube design that the apparatus described herein is directed.

SUMMARY OF THE INVENTION

Embodiments of the firetube heat exchanger described herein comprise an elongated cylindrical shell having a fluid inlet end, a fluid outlet end and a fin assembly secured on the inner surface of the shell. The fin assembly comprises a plurality of circular rows of elongated U-shaped fins, each fin having a bottom surface secured to the inner surface of the shell with two generally flat, planar sides extending upwardly from the bottom fin surface. The fins in each row are aligned substantially parallel along the axis of the cylindrical shell, and the fins of one or more rows of fins may be offset angularly from the fins of an adjacent row of fins. In some embodiments, the flat, planar sides of the fins are substantially parallel and the fins in each row of fins, respectively, are substantially identical in fin height, length and width. In other embodiments, the dimensions of fins in at least two of the rows are different in height, and/or width, and/or length. In yet another embodiment, three or more different fin heights are used within the firetube heat exchanger. These as well as other variations in designs and embodiments of the fins and the firetube heat exchanger design will be described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of one embodiment of a firetube showing a semi-transparent cylindrical tube shell.

FIG. 2 is an cross-sectional isometric view taken across the line 2-2 of one embodiment of a firetube that illustrates the interior fin arrangement and design.

FIG. 3 is an end view of a fin and illustrates three different fin heights.

FIG. 4 is a side view of a fin shown in FIG. 3, also illustrating three different fin height designations.

FIG. 5 is a cross-sectional view of the split firetube of FIG. 2 with an interior ceramic core plug installed.

DETAILED DESCRIPTION

One embodiment is a firetube heat exchanger that includes an outer shell. Disposed along the interior surface of the shell is a fin assembly having a plurality of circular rows of elongated U-shaped fins. In one embodiment, each fin has a bottom surface that is secured to an inner surface of said cylindrical shell. Each fin may also have two sides extending upwardly from said bottom surface and defining an elongated interior channel. The sides may be planar and flat. In addition, in one embodiment, the fins in each row may be aligned substantially parallel along the axis of the cylindrical shell. In one embodiment, the sides of the fins in different rows have differing heights.

In FIG. 1 a firetube heat exchange assembly 10 is illustrated with the cylindrical shell 11 shown in semi-transparency for viewing the interior fins. Reference is also made to the cross-sectional view of FIG. 2 in which the cylindrical shell has been sectioned to show more particular features of the fin assembly.

As illustrated, the fin assembly is secured circumferentially around the inner surface of the cylindrical shell and comprises a plurality of circular rows of elongated U-shaped fins. In the illustrated embodiment, the fins in each row, respectively, are substantially identical and using fins of three different heights in different rows of fins. The first row of fins nearest to the fluid inlet end 13 of shell 11 comprises substantially identical fins 12, the second row comprising substantially identical fins 14 and the third row and the remaining rows made up of substantially identical fins 16. In this embodiment, the difference between fins 12, 14 and 16 is in the height of their upwardly extending sides. In this embodiment, the fin sides are lower in the front of the firetube where gas temperatures are hottest.

The length of the fins of all the rows may be the same, although different fin lengths in the different rows may be used. However, all of the fins in any single row may have substantially the same length. Similarly, the width of the fins in any row may be the same, although different fin widths may be used. However, in some embodiments, all of the fins in a row have substantially identical widths. In other embodiment, all of the fins in all of the rows of the firetube have substantially identical widths.

The difference in the heights of the sides of the fins of the different rows is further illustrated in FIGS. 3 and 4. The heights of opposite sides 22 and 24 of fin 20 are the same. However, the upper edge 21 of all fins 12 in the first row of fins is shorter than the height of the sides of the fins in rows 14 and 16. Specifically, the upper edge 23 of the sides 22, 24 of all second row fins 14 is greater than the height of the fins in row 12 and shorter than the height of fins in the third row of fins 16 and the remaining rows of fins all having an upper edge 25.

In one embodiment, the height of the fins differs by between 10% and 50%. In another embodiment, the height of the fins differs by between 15% and 35%. In yet another embodiment, the height of the fins differs by between 20% and 30%. In one embodiment, one row of fins is 0.5 inches tall, the second row of fins are ⅝ inches tall and the fins in the third and remaining rows are 0.75 inches tall. In one embodiment, each row of fins from the first row to the third row is 25% taller than the preceding row.

As previously described, and illustrated particularly in FIG. 3, all of the fins have substantially the same width and are U-shaped with a bottom surface 26. The bottom fin surface is generally flat or is arched or curved preferably on a radius (radiused) to better match the radius or curvature of the inner cylindrical surface of the shell underlying the bottom fin surface. Such a radiused bottom surface will also facilitate brazing of the fin and cylinder surfaces. Such a flat or curved bottom also provides a surface for tack welding or spot welding each fin in place during assembly of the firetube heat exchanger.

In another embodiment, the opposite fin sides are parallel and extend upwardly substantially perpendicular (normal) from the bottom surface. However, the opposite sides may also be somewhat angled at obtuse or acute angles from the bottom surface. Such angles may be selected, depending on the desired number of fins in each row, as well as the desired spacing of the fins in each row. It will also be understood that the specific number of fins in each row will depend on the width of the fins and the radial dimensions or circumference of the cylindrical shell.

In this embodiment, the fins in each respective row are aligned lengthwise with their upwardly extending sides aligned substantially parallel along the axis of the cylindrical shell. As previously noted, the shortest fins or fins in rows of fins are at the inlet end of the firetube, and fins in succeeding rows have higher sides. The specific number of different heights of fins in the firetube may be selected, but at least two different heights may be used. In another embodiment, at least three different heights of fins are used, although more different heights may also be used without departing from the invention. In the embodiment illustrated, three different heights of fins are used, as previously described and shown in FIGS. 1-4.

The fins in adjacent rows of fins may be aligned angularly along the length of the firetube or fins of adjacent rows of fins may be offset angularly from one another. Of course, if the fins of adjacent rows of fins are of different widths, the upwardly extending sides of the fins in adjacent rows will present an offset of fin sides from inlet to outlet along the length of the firetube. In one embodiment, with the fins being of substantially the same width, the fins may be aligned angularly without offset, or they may be offset angularly up to one-half of the fin width.

The specific number of rows of fins will depend on the length of the firetube, and the length of the fins in the different rows of fins. The number of rows of fins of between 2 and about 20 rows is preferred and more preferred is between about 4 and about 12 rows of fins, fewer fins results in more heat stress along the firetube. By way of example, for a firetube of about 2 feet in length, 10 rows of fins having an equal fin length in each row is shown in the drawings.

The upper edges of the upwardly extending fins defines an elongated interior channel in which is secured a heat resistant insert, often referred to as a core plug, and which is typically made of a heat resistant ceramic material. The length of the insert may extend between the second row of fins from the inlet end and the last rows of fins at the outlet end, as illustrated in FIG. 5. The shape of the insert is such that the diameter gradually increases from the forward end, closest to the fluid inlet of the firetube, leaving a space between the surface of the insert and the upper edges of the fin sides for a portion of its length in and then contacting the fin edges along a successive portion of the firetube length. Such shape of the insert, its dimensions, and placement are well understood by those skilled in the art.

In another embodiment, the firetube heat exchanger assembly includes copper rings extending between rows of fins and the firetube surface. The copper rings may be mounted between all rows of fins, with each ring contacting the interior of the surface of the firetube as well as the ends of fins in adjacent rows. At least one ring may be mounted at the end of the last row of fins. In another embodiment, a plurality of copper rings is mounted at the end of the last row of fins. In FIG. 2, copper rings 30, 31, 32, 33, 34, 35 are illustrated. The copper rings are shown only between every other row of fins by way of example and for simplicity, but again, a ring may be disposed between every row of fins. The copper rings may be mounted using vacuum brazing or brazed in a hydrogen furnace, or otherwise installed by brazing techniques known to those skilled in the art.

In one embodiment, the rings comprise high purity (above 98%) copper because of its ductility and conductivity. However, the use of mixtures of copper with another conductive metal, for example nickel, is not precluded. It is to be understood that when the copper ring is brazed, it will melt and flow to both rows of fins and the tube interior surface creating a conductive and ductile bond therebetween. Since the rings are to be brazed, their cross-sectional shape prior to brazing is not critical.

The firetube heat exchanger described herein is useful in any heat exchange apparatus for directing heat from hot gases of combustion passing along the inside of the firetube to heat liquids contacting the outside surface of the firetube. The firetube is especially useful in a boiler or stripping section of the generator of an aqua-ammonia absorption system, for example, a GAX absorption system, such as described in U.S. Pat. Nos. 6,487,875, 6,427,478, 6,718,792, 6,735,963 and 6,748,752. The firetube heat exchanger described herein has advantages of being cost effective to manufacture, reliable, and efficient as compared to other firetubes used and known in the prior art. 

1. A firetube heat exchanger comprising: an elongated cylindrical shell having an inlet end and an outlet end; and a fin assembly secured on the inner surface of said shell, said fin assembly comprising a plurality of circular rows of elongated U-shaped fins, each fin having a bottom surface secured to the inner surface of said cylindrical shell and two flat, planar sides extending upwardly from said bottom surface and defining an elongated interior channel, wherein said fins in each row are aligned substantially parallel along the axis of said cylindrical shell.
 2. A firetube heat exchanger of claim 1 wherein the bottom surface of each of said fins comprises a generally flat, planar surface.
 3. A firetube heat exchanger of claim 1 wherein the bottom surface of said fins is curved on a radius.
 4. A firetube heat exchanger of claim 1 wherein the fins in each row of fins, respectively, are substantially identical.
 5. A firetube heat exchanger of claim 1 wherein the height of the fins in the first row of fins adjacent to the inlet are lower than the height of fins in other rows of fins.
 6. A firetube heat exchanger of claim 5 comprising three or more rows of fins and wherein the height of fins in the second row of fins from the inlet end are higher than the fins of said first row of fins and lower than the height of fins of one or more successive rows of fins.
 7. A firetube heat exchanger of claim 5 comprising between 2 and 20 rows of fins.
 8. A firetube heat exchanger of claim 5 comprising between 4 and 12 rows of fins.
 9. A firetube heat exchanger of claim 6 comprising between 4 and 12 rows of fins.
 10. A firetube heat exchanger of claim 1 wherein fins of one or more rows of fins are offset angularly from the fins of an adjacent row of fins.
 11. A firetube heat exchanger of claim 1 wherein the height of the sides of all fins in a row of fins are equal.
 12. A firetube heat exchanger of claim 1 wherein the width of all fins in a row of fins are equal.
 13. A firetube heat exchanger of claim 1 wherein the width of all fins are equal.
 14. A firetube heat exchanger of claim 11 wherein the width of all fins are equal.
 15. A firetube heat exchanger of claim 10 wherein the height of the sides of all fins in a row of fins are equal.
 16. A firetube heat exchanger of claim 10 wherein the width of all fins in a row of fins are equal.
 17. A firetube heat exchanger of claim 10 wherein the width of all fins are equal.
 18. A firetube heat exchanger of claim 13 wherein the fins of adjacent rows of fins are offset angularly up to one-half of the fin width.
 19. A firetube heat exchanger of claim 18 wherein the height of the sides of all fins in a row of fins are equal.
 20. A firetube heat exchanger of claim 18 wherein the height of the fins in the first row of fins adjacent to the fluid inlet are lower than the height of fins in other rows of fins.
 21. A firetube heat exchanger of claim 20 wherein the height of the sides of all fins in a row of fins are equal.
 22. A firetube heat exchanger of claim 1 wherein the length of all fins in a row of fins are equal.
 23. A firetube heat exchanger of claim 1 wherein the length of all fins in a row are equal and the lengths of fins in two or more different rows are different.
 24. A firetube heat exchanger of claim 1 wherein the length of all fins are equal.
 25. A firetube heat exchanger of claim 24 wherein the height of the sides of all fins in a row of fins are equal.
 26. A firetube heat exchanger of claim 24 wherein the width of all fins in a row of fins are equal.
 27. A firetube heat exchanger of claim 24 wherein the width of all fins are equal.
 28. A firetube heat exchanger of claim 24 wherein the fins of adjacent rows of fins are offset angularly up to one-half of the fin width.
 29. A firetube heat exchanger of claim 24 wherein the height of the sides of all fins in a row of fins are equal.
 30. A firetube heat exchanger of claim 24 wherein the height of the fins in the first row of fins adjacent to the fluid inlet are lower than the height of fins in other rows of fins.
 31. A firetube heat exchanger of claim 4 wherein the height of the fins in the first row of fins adjacent to the fluid inlet are lower than the height of fins in other rows of fins.
 32. A firetube heat exchanger of claim 31 comprising between 4 and 20 rows of fins and wherein the height of fins of row 3 and subsequent rows of fins are equal.
 33. A firetube heat exchanger of claim 32 wherein fins of one or more rows of fins are offset angularly from the fins of an adjacent row of fins.
 34. A firetube heat exchanger of claim 32 wherein the fins of adjacent rows of fins are offset angularly up to one-half of the fin width.
 35. A firetube heat exchanger of claim 1 wherein the two sides of each of said fins are substantially parallel.
 36. A firetube heat exchanger of claim 2 wherein the two sides of each of said fins are substantially parallel and perpendicular to the bottom surface.
 37. A firetube heat exchanger of claim 2 wherein the two sides of each of said fins extend upwardly from said bottom surface at acute angles or obtuse angles.
 38. A firetube heat exchanger of claim 3 wherein the two sides of each of said fins are substantially parallel.
 39. A firetube heat exchanger of claim 3 wherein the two sides of each of said fins extend upwardly from said bottom surface at acute angles or obtuse angles.
 40. A firetube heat exchanger of claim 1 further comprising a heat resistant insert positioned concentrically along a portion of the length of said interior channel.
 41. A firetube heat exchanger of claim 1 further comprising a plurality of brazed thermally conductive rings forming a conductive and ductile bond between the inner surface of said cylindrical shell and adjacent rows of said fins.
 42. A firetube heat exchanger of claim 41 comprising a said brazed thermally conductive ring between all adjacent rows of said fins.
 43. A firetube heat exchanger of claim 41 comprising one or more brazed thermally conductive rings along the inner surface of said cylindrical shell between the last row of fins and said fluid outlet end.
 44. A firetube heat exchanger comprising: an elongated shell having an inlet end and an outlet end; and a fin assembly secured on the inner surface of said shell, said fin assembly comprising a first and second row of U-shaped elongated fins, each fin having at least one planar side extending upwardly from said inner surface and defining an elongated interior channel, wherein said first row of fins is adjacent said inlet end and wherein said second row of fins is taller than said first row of fins.
 45. A firetube heat exchanger of claim 44, wherein each row of elongated fins are aligned substantially parallel along a central axis of said cylindrical shell. 